1. Field
The invention relates to the field of underground storage tanks, particularly for storage of hazardous liquids such as gasoline, and to structures for housing such storage tanks.
2. State of the Art
Underground storage tanks are used for storage of a variety of hazardous liquids such as gasoline, oil, diesel fuel, other toxic substances, and chemicals. Such tanks are most often used in gas stations, truck stops, and industrial and commercial facilities. Most of these tanks have capacities between 500 and 20,000 gallons and are made of unprotected, welded sheet steel which is subject to corrosion and other forces when buried. Such tanks are likely to leak after being buried for a number of years. More recently some tanks have been made of fiberglass, but such tanks are subject to cracking or other problems which cause these tanks to leak also.
In the event of a leak developing in a tank, the gasoline or other hazardous fluid will leak into the ground. Leakage may occur from such tanks over a relatively long period of time prior to detection. This pollutes the ground and usually leads to extremely costly cleanup operations once the leak is discovered. With existing underground tanks an average of twenty-five years old, and with the risk of leaks increasing substantially after about twelve years in the ground, it is currently estimated that up to about twenty percent of underground storage tanks are probably leaking.
Because of the growing environmental problems associated with leaking underground storage tanks, the U.S. Environmental Protection Agency has recently adopted new regulations requiring regular leakage testing of underground storage tanks and the carrying of insurance policies to cover the costs of any required environmental cleanup. Complying with these new regulations significantly adds to the cost and responsibility of owning and operating underground storage tanks. In many cases, the required insurance, if available at all, is so expensive independent gas station owner cannot afford it. The Environmental Protection Agency's new regulations do not apply, however, when the storage tanks themselves are not buried but are placed within a structure where they can be inspected and where any leakage can be contained. Hence, placing storage tanks within a structure, either above ground or under ground, is a way to both avoid EPA regulations and prevent environmental problems.
U.S. Pat. No. 4,638,920 discloses a steel enclosure or vault built around a tank. The enclosure and tank, as a unit, is then lowered into the ground. With such an enclosure unit, size and weight can be a problem in terms of transportation and lowering of the unit into the excavation. Further, since the enclosure is made of steel, it is subject to the same corrosion activity as the tank would be.
Prefabricated concrete enclosures or vaults for storage tanks are available from SCV Corporation under the trademark "Surevault". These enclosures are manufactured in sections and assembled on site. The enclosures are made up of an open top, bottom section with walls which extend upwardly so as to form a structure which will hold the entire contents of a tank located therein should the tank completely fail. A precast collar section is placed on top of the bottom section walls to form the upper walls, and deck panels are placed thereon to cover the top. As with the steel enclosure of U.S. Pat. No. 4,638,920, only certain maximum sizes of prefabricated units can be made for transportation and, because the bottom section include partial walls, the size of the enclosures are limited to holding a single tank. For multiple tanks, multiple enclosures are used.
Swiss patent No. 466,551 similarly shows a tank enclosure made up of lower and upper prefabricated sections that are joined together to form the enclosure.
Japanese Patent No. 57-158436 shows a method of constructing a concrete underground room. No particular use is shown for the room. The walls are assembled from precast concrete panels and the excavation is done below these walls to lower the walled structure to the desired depth. When at the desired depth, the floor is formed.
There remain a need for a structure made of precast components which can be easily assembled at a job site into a structure for housing storage tanks, which is sealed to be fluid proof, and which can be assembled in various different sizes so as to accommodate various numbers of tanks.